1. Field
This disclosure relates generally to image sensors, and in particular but not exclusively, relates to reducing noise in image sensors.
2. Background Information
Image sensors are prevalent. They are widely used in digital still cameras, digital video cameras, camera phones, picture phones, video phones, camcorders, webcams, computer system cameras, security cameras, medical imaging devices, optical mice, toys, games, scanners, automotive image sensors, as well as other types of electronic image acquisition devices.
One significant factor that may limit the quality of the images and/or video acquired by an image sensor is the amount of noise that is present. Various factors may contribute to noise, such as variations in temperature, fluctuations or noise in electrical signal, variations in circuitry of the image sensor, etc.
Correlated double sampling (CDS) is a known noise reduction technique commonly used in image sensors to help reduce the amount of noise and improve image and/or video quality. CDS may help to reduce certain types of noise such as fixed pattern noise (FPN), reset noise, thermal noise in capacitors or capacitive structures referred to as kTC noise, and flicker (1/f) noise.
FIG. 1 is a block diagram illustrating a known CDS technique. As shown in the illustration, a reset level of a given pixel may be read at a first time 101. The reset level may represent a voltage level on a column readout line or bit line after the given pixel has been reset and while the given pixel is still in the reset state.
As further shown, an image signal level of the same given pixel may be read at a second, different time 102. The image signal level may represent a voltage level on the column readout line or bit line when the image data from the same pixel is transferred to the column line or bit line for readout.
The reset level of the given pixel read at the first time 101, and the image signal level of the same given pixel read at the second, different time 102, are provided to a correlated double sampling (CDS) circuit 103. The CDS circuit subtracts the reset level of the given pixel read at the first time from the image signal level of the same given pixel read at the second, different time. The subtraction of these levels helps to cancel or reduce part of the noise that is “correlated” between the reset and signal samples. The CDS circuit outputs a reduced noise image signal 104.